Groundwater is a critical water resource across the US and world-wide. Its natural interactions with surface water help stabilize our rivers and ecosystems. Despite its central role in the water balance, groundwater is hard to see, leading to much uncertainty on its availability. As a result, it is simply characterized in global models, and often misunderstood outside the hydrologic community. Though the need for better accounting of groundwater storage in global models has been widely acknowledged, there are still critical gaps in our understanding of how groundwater storage functions across large spatial scales and over long time periods. This uncertainty must be addressed to rigorously evaluate new modeling approaches and improve the hydrologic projections we make with global models. This project explores the role of groundwater storage in large scale models and develops tools to validate groundwater behavior in large coupled systems. The project will further develop ways to improve effective communication about groundwater to the public. We will use our modeling platforms as a resource to engage K-12 students in computer science with locally relevant water issues. The proposed research will quantify the role of large-scale storage dynamics in water and carbon cycles, which is an identified weakness of our current earth systems projections. More specifically, this project seeks to understand how storage dynamics control hydrologic regimes, and what role storage changes could play in future hydrologic regime shifts. Research will leverage the first fully integrated groundwater surface water model of the Continental US to develop direct comparisons with other large-scale modeling approaches and simplified groundwater representations over large spatial extents at varying spatial resolutions. This will provide unique insights on the impact of process representation and spatial resolution on global water and carbon projections not possible with previous approaches. The key outcomes of this research will (1) characterize the most sensitive hydrogeologic settings for simplification of groundwater approximations and spatial resolution, (2) map spatiotemporal ?groundwater regimes? across the US and analyze spatial controls, (3) analyze existing global model storage benchmarks, (4) quantify relationship between groundwater configuration and response to climate change, and (5) identify regions that are the most vulnerable to groundwater storage changes. In parallel with these technical outcomes this project will improve public understanding and engagement around groundwater sustainability. Broadcast meteorologists already regularly communicate hydrologic information with the general public. This project will provide training and broadcast resources on groundwater systems to this group, reaching a much larger and more diverse audience than would be possible through direct public outreach. Undergraduates at the UA will help develop broadcast materials through existing science communication courses, thus also providing exposure to hydrology for students outside of hydrology. Finally, the simulation platforms used here will be leveraged as a tool to engage high school students in local watershed issues while learning computer science and coding skills. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.